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Green Olympic pool: perfect storm of sun, heat and still water?

Spot the difference

Sylvain Marchandise/Reuters

By Andy Coghlan

What has happened to the Olympic diving pool? Divers, commentators and spectators were left baffled yesterday when the diving pool at the Maria Lenk Aquatics Centre in Rio suddenly turned murky green in time for the women’s 10m final. A combination of sunlight and low chlorine circulation could be to blame.

Some have suggested that the green colour has been caused by corrosion of pipes, releasing copper or iron ions into the water. But if this is the case, why has the swimming pool next to it remained blue?

Alternatively, algae or bacterial spores could be to blame. The diving pool is kept at a warmer temperature than the swimming pool, meaning green photosynthetic organisms are more likely to grow in it. Normally chlorine would stop this from happening, but it hasn’t seemed to here.

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This might be because this is the first Olympics for many years that has used pools that are outdoors. Unlike in previous competitions, the pool water has been exposed to sunlight and ultraviolet light, which can break down chlorine-based disinfectants, and cause them to evaporate.

“It needs the chlorine disinfectant to be clear, bright, blue and sparkling, so if it’s not there, the water could become discoloured,” says Ralph Riley, at the UK Pool Water Treatment Advisory Group.

Calm waters

An additional factor that may explain the difference between the two pools is the fact that, unlike swimmers, divers require a calm, completely undisturbed pool to target their dives. This may mean that the pumps that normally circulate disinfectants throughout the pool had been turned off for prolonged periods during the competition, further depleting the chlorine levels in the diving pool.

“To keep water clean, you must keep it circulating, filtered and disinfected,” says Riley. “Normally, the water turns over completely every six hours, but with a diving competition, they may turn it off for longer so the surface is not disturbed,” he says.

The water quality at the Maria Lenk Aquatics Centre was tested and there were no risks for the athletes. We are investigating the cause.